The present invention relates to improvements of a semiconductor integrated circuit including a diode and a system LSI.
In recent years, development of system LSIs having higher functions has entailed coexistence of a plurality of digital circuit blocks and a plurality of analog circuit blocks in one chip. This configuration produces such a problem that noise from the digital circuit blocks propagates to the analog circuit blocks. To avoid this in the system LSI, the power supply of the digital circuit blocks and the power supply of the analog circuit blocks are separate from each other. However, for the purpose of protection against damage due to electro static discharge (ESD), a single unit of protection device formed by a pair of diodes D1 and D2 placed in opposite directions is inserted between the power supply of the digital circuit blocks and the power supply of the analog circuit blocks as shown in FIG. 20. Insertion of these diode elements D1 and D2 can reduce the propagation of power supply noise and can ensure surge resistance to ESD.
A specific structure of such diode elements D1 and D2 provided for the purpose of reducing noise propagation is realized by MOS transistors as disclosed in, for example, Japanese Patent No. 2598147.
With respect to reduction in area, a desirable specific structure of the diode elements D1 and D2 is formed by gate-less diode elements instead of MOS transistors used in the conventional structure.
A proposed structure of such a diode element is, for example, one shown in FIG. 21. The diode element D shown in FIG. 21 includes, over a well 1, a diffusion layer 5 which constitutes the anode and diffusion layers 4 provided on the left and right sides of the diffusion layer 5 which constitute the cathode. These diffusion layers 4 and 5 are both formed in a pattern of rectangular strips. Around the diffusion layers 4 and 5 constituting the anode and cathode, a well contact 2 is provided to surround the diffusion layers 4 and 5 constituting the anode and cathode such that noise from the anode and cathode does not propagate to the outside. The diffusion layers 4 and 5 constituting the anode and cathode and the well contact 2 are provided with a large number of contact holes 3 formed thereover for connection with the outside.
When a surge voltage is applied to such a diode element D, a current is allowed to flow from the diffusion layer 5 constituting the anode to the diffusion layers 4 constituting the cathode as illustrated by solid lines in FIG. 22, whereby the diode element D is discharged and absorbs the surge voltage.
However, it was found that the above-described structure causes the following problem. Namely, since the diode element D shown in FIG. 21 is surrounded by the well contact 2, a current flows from the diffusion layer 5 constituting the anode to neighboring part of the well contact 2 as illustrated by broken lines in FIG. 22 in addition to the current flowing from the diffusion layer 5 constituting the anode to the diffusion layers 4 constituting the cathode. In this case, it is expected that the current converges at contact holes 3 of the anode diffusion layer 5 in the vicinity of the well contact 2 to cause breakage of the contact holes 3.
Such an undesirable result might be avoided by employing such a structure that, for example, the four corners of the diffusion layer 5 constituting the anode are truncated, and the contact holes 3 of the diffusion layer 5 are located distant from the four corners, such that electric field convergence at the corners is reduced. However, on the other hand, this structure would result in a smaller number of contact holes. Since the largeness of the current which flows through the diode element is limited by the number of contact holes, the diode element cannot produce a sufficient outcome with respect to its capacity as a protection device. This especially matters in microstructure processes performed with small contact hole diameters.